Gameplay.
Very few and I mean VERY few stories, as hard as they try, are actually captiviating. Recently, the only story that I've actually payed attention to is Oblivion; even then, only the voice acting and star power made it so. Guess I'll throw in HL2: Episode 1 and 2. Bioshock was cool, but honestly, I did not really care until the twist near the end. The other 99% of the time, it's the gameplay. I love Stalker and Medieval 2 cause of their gameplay, not their story, which is sad. Way back when, Baldur's Gate, BG II + exp, Icewind Dale and Deus Ex actually made me care. Maybe I've been jaded or stories just suck nowadays.